The present invention generally relates to fabrication of semiconductor devices and liquid crystal display devices, and more particularly to a process and apparatus cleaning used in a fabrication process of semiconductor devices or liquid display devices, for cleaning a substrate.
In the fabrication of semiconductor devices or liquid crystal display devices, a cleaning process for cleaning a semiconductor substrate or glass substrate is essential. Such a cleaning process generally includes a chemical treatment process in which the substrate is immersed in a liquid of chemicals for surface treatment and a rinse process, using a large amount of pure water, for removing the film of the chemical liquid from the surface of the substrate which is applied after removing the substrate from the cleaning liquid. It has been known conventionally that the rinse effect is enhanced substantially by using warm pure water. In such a rinse process, it is also important that deposition of dust particles on the cleaned Substrate surface is minimized.
In order to avoid deposition of dust particles in the rinse process, it is proposed, in the Japanese Laid-open Patent Publication 63-155729, to maintain the temperature of the pure water used in the rinse process at a low temperature such as 5.degree. C. and introduce a low temperature gas such as liquid nitrogen into the pure water so as to cause a freezing of the dust particles. According to the foregoing conventional process, the dust particles act as nuclei of icing, and the dust particles thus surrounded by ice are removed from the substrate surface. However, such a process, while requiring an extensive facility, cannot achieve the desired rinse effect due to the low temperature of the pure water used for rinsing.
The inventor of the present invention has conducted a series of experiments about the rinse effect achieved by warm pure water, and found an interesting relationship between the temperature of the pure water used for rinse and the number of dust particles remaining on the substrate. Hereinafter, the referred to experiments and the result will be described in brief.
FIG. 1 shows the procedure of the experiment.
Referring to FIG. 1, a silicon wafer of 8 inches diameter was prepared for the experiment in a step 1, and the number of the dust particles having a grain size larger than 0.2 .mu.m was measured in a step 2. Next, in a step 3, the wafer was immersed in a cleaning liquid that was a mixture of 32% H.sub.2 O.sub.2 and 98% H.sub.2 SO.sub.4 in a volumetric ratio of 2:100. Next, the wafer was pulled up from the chemical liquid and subjected in a rinse process in step 4, wherein the rinse process was conducted for 30 minutes using pure water of various temperatures. After 30 minutes, the rinse was continued in a step 5 in a vessel while causing an overflow of the pure water. The substrate was subsequently dried in a step 6 for 20 minutes, and the surface of the wafer 8 was scanned in a step 7 for detection of particles having a grain size exceeding 0.2 .mu.m. By analyzing the results of this measurement, in a step 8, the number of the remaining particles were obtained.
FIG. 2 shows the result of the experiment of FIG. 1.
Referring to FIG. 2, it will be noted that the number of the dust particles remaining on the substrate after the rinse process increases with increasing temperature of the pure water used for the rinse process. Although the reason of this tendency is not entirely understood, it is concluded that the number of the dust particles on the substrate exceeds the allowable limit of 15 particles when the temperature of the pure water is set higher than 25.degree. C. in the rinse process. It should be noted that the foregoing 15 particles on the substrate is the allowable limit for an 8-inch substrate. Meanwhile, it is also known that the effect of rinse tends to become insufficient when the temperature of the pure water used for rinse is low.
FIG. 3 shows the procedure of an experiment conducted by the inventor for evaluating the relationship between the temperature of the pure water rinse and the rinse effect.
Referring to FIG. 3, the experiment was started by preparing an 8-inch wafer of Si in a step 11, followed by a dipping process in a step 12 for dipping the 8-inch diameter substrate into the foregoing cleaning liquid, which is a mixture of H.sub.2 O.sub.2 and H.sub.2 SO.sub.4 similarly as before. Next, a rinse process was conducted in a step 13 in pure water for 10 seconds, while using various temperatures for the pure water, ranging from room temperature to 60.degree. C. Further, the specific resistance of the wafer was measured in a step 14 while holding the wafer in a pure water overflow. Further, the wafer thus rinsed was dried in a step 15.
FIG. 4 shows the result the measurement of the specific resistance, wherein the vertical axis represents the time needed for recovering the specific resistance of the pure water and can be interpreted as representing the amount of the acid remaining on the surface of the substrate. As will be seen clearly from FIG. 4, the time needed for restoring the high specific resistance inherent to the pure water decreases with increasing temperature of the pure water used for the rinse. Thus, the tendency of FIG. 4 confirms the conventionally accepted knowledge that the rinse effect tends to become insufficient when a low temperature pure water is used for the rinse.
Summarizing above, it is concluded that the number of the dust particles remaining on the substrate increases when the temperature of the pure water is set high for better rinse effect, while use of low temperature pure water for minimizing the deposition of dust particles on the substrate invites an insufficient rinse effect. When the rinse effect is insufficient, chemicals used for the surface treatment of the substrate remain on the substrate surface, and the fabrication processes of the semiconductor device conducted subsequently are affected unwantedly.